1. Field of the Invention
The field of the invention is that of heterogeneous networks, including at least one basic network and at least one sub-network.
2. Description of the Prior Art
We shall now briefly recall the technical approach currently used to synchronize a destination terminal with a data stream in the context mentioned here above, namely:                when the data stream is transmitted from an entry terminal to a destination terminal, these terminals being connected to heterogeneous network of digital buses, and        when this stream crosses the basic network, from an entry node to at least one destination node of this basic network.        
It is assumed that the digital buses convey first packets (for example IEEE 1394 packets) and the basic network conveys second packets (for example IEEE 1355 packets). Thus, the data stream concerned is conveyed on the digital buses by first packets. To enable the crossing of the basic network by this data stream, the entry node divides and/or concatenates these first packets to encapsulate them in second packets. Conversely, the destination node de-encapsulates the contents of the second packets that it receives, and then generates first packets.
Typically, the basic network is a switched network conveying second packets whose size is variable and determined by quality of service (QoS) imperatives. Since the second packets are of variable size, it is impossible to ensure an alignment of the first and second packets.
In the current technique of synchronization, in the case of a point-to-point connection, first the destination terminal and then the sender terminal is initialized. Here below the term “initial first packet” is applied to the first (in terms of rank) packet of the first (in terms of type) packets. With the current technique, the sender terminal therefore sends the initial first packet only when the destination terminal is ready to receive it. Thus, during the encapsulation performed by the entry node, this initial first packet is matched with the start or beginning of a second packet. After the de-encapsulation performed by the destination node, there is no ambiguity whatsoever on the boundary of the initial first packet. The destination terminal therefore receives an initial first packet that is strictly identical to the initial first packet generated by the entry terminal. In processing the header of this initial first packet, the destination terminal is in a position to retrieve all the other first packets that it receives thereafter.
A first drawback of the current technique mentioned here above is that it relies on a relatively cumbersome protocol, since the destination terminal has to be initialized before the entry terminal.
Another drawback of the current technique mentioned here above is that, by virtue of its very principle, it does not allow for the successive synchronization of several destination terminals on one and the same data stream. Thus, it is impossible to apply this current technique in the case of a multicast stream, where a first destination is synchronized with the stream, then one or more destination terminals must be capable of getting synchronized with this same stream at any point in time. Indeed, after the contents of the second packets received have been de-encapsulated, each destination node to which one of these other destination terminals is connected (directly or through a digital bus) generates first packets. However since the beginning of a second packet, barring accidents, is not matched with the beginning of one of the first packets generated by the entry terminal, the first packet generated by the destination node does not coincide with first packets generated by the entry terminal. In other words, the destination node is not synchronized with the data streams since the boundaries of the beginning of the first packets generated by the destination node do not coincide with the boundaries of the beginning of the first packets generated by the entry terminal. Owing to this gap, the destination terminal is never in a position to get synchronized with the stream and therefore to recover all the other first packets that it receives thereafter.
FIG. 10 illustrates the segmentation and scheduling mechanism in order to send first packets containing IEEE1394 isochronous data over the basic network using second packets, according to the prior art.
As described previously, the second packets 1000 are built in order to satisfy a quality of service when transferring data over the basic network. Their size and scheduling depend on the network conditions, and may vary over the time. In addition the transported IEEE1394 isochronous traffic may also vary over the time, as it is application dependent. Thus most of the time there is no match between the boundary of a second packet 1000 and a first packets 1001.
Considering the case where a data stream connection is already established between an entry node of the basic network and a destination node of the basic network. Another destination node of the basic network may request to be connected to that stream resulting into a multicast communication. Without the invention, the secondly connected destination node will start the reception of the data stream in an unpredictable manner and won't be able to retrieve the original IEEE1394 isochronous packets (first packets) 1001 as it won't be able to find the original packet boundary in the basic network packets (second packets) 1000.